Solving
the St. Louis Formation Reservoir Puzzle in Kansas: Application of a Regional
Carbonate Cyclostratigraphic and Diagenetic Reservoir Model for Exploring
in Mature Areas

Ursula
Hammes and James Lakings

Abstract:

A cycle
stratigraphic and diagenetic framework for the Mississippian St. Louis
Formation in southwest Kansas was established to explain reservoir heterogeneities,
trap configurations, and depositional and diagenetic patterns. Cyclicity
during St. Louis time was driven by high-frequency, low-amplitude, sea-level
fluctuations associated with the beginning of glaciation of Gondwanaland.
Four fourth-order cycles punctuate the stratigraphic record of the St.
Louis Formation. Generally, each upward-shallowing cycle starts with tidal-influenced,
siliciclastic-rich grainstones (transgressive systems tract) overlain
by lagoonal wackestones (maximum flooding surface), which are capped by
skeletal ooid grainstones (highstand system tract). Skeletal grainstones
and localized, dolomitized packstones and wackestones form the main reservoir
compartments in most St. Louis fields. Identifying and correctly correlating
these individual cycles contributed to the success of several discoveries
and delineation of a waterflood in one field in southwest Kansas. Associated
diagenetic environments are characterized by marine, meteoric phreatic,
and shallow burial cements, which occlude primary pore spaces. Ooid grainstones,
deposited in the center of paleohighs, exhibit a greater degree of cementation
than the skeletal grainstones related to higher abundance of unstable
aragonite mineralogy of predominantly ooids. In contrast, skeletal grainstones,
located around the periphery of paleohighs, are mostly low-Magnesium calcite
grains, which were stable under near-surface conditions and preserved
significant primary porosity. Deeper burial dissolution affected ooid
grainstones in creating microporosity in ooids and solution-enlarged pores
in skeletal grainstones, thus enhancing porosity. Drilling slightly off
the paleostructural high resulted in a successful well with good porosity
in skeletal grainstones.